High-resolution optical mapping with voltagesensitive dyes has become a powerful tool to depict complex propagation patterns of cardiac transmembrane potentials. Many studied have proved that this optical signal obtained from tissue surface is the response of the transmembrane potential averaged upon depth rather than only surface. In order to investigate the differentia between the two transmembrane potentials, in this paper, we simulated cardiac propagation using Luo-Rudy (L-R) model and calculated the transmembrance potentials average over depth by an optical decay constant from 0.0 mm to 3.0mm. Our results suggest that the transmembrance potentials weighted average over depth is different from that on the tissue surface, the discrepancy between them depends on the depth of the fluorescence emission of the tissue. If only top layer of tissue (<1.0mm) contributes the fluorescence, optical mapping is an almost accurate representation of surface activation dynamics.